This invention relates generally to vacuum bag methods of producing fiber reinforced polymer matrix composite parts, and, more particularly, to vacuum bag methods capable of accurately controlling part thickness and producing parts having a low volume fraction of fiber reinforcement.
The volume fraction of reinforcement fiber in a composite is a variable which composite fabricators seek to control since it does much to determine the physical properties of the composite. It is most often desirable to have a high volume fraction of fiber so that the composite will be very strong and stiff. However, there are other applications, such as a skirt to cover the tracks of an armored vehicle, for which it is desirable to have a relatively low fiber volume percentage so that the part can be flexible. Furthermore, there are also occasions when it is necessary to control the thickness of a composite part very carefully.
Composites made by standard vacuum bag methods, using either wet resin lay-ups or prepreg technology and typical reinforcement fabrics, will usually contain about 50% fiber by volume. This percentage can be increased by the application of pressure using standard composite processing equipment such as a compression press or an autoclave during the cure cycle, i.e., the period of time in which the matrix resin reacts so that the viscosity increases from that of a liquid to that of a solid. Alternatively, a composite fabricator can place a bleeder fabric between the wet part and the vacuum bag to bleed out excess resin through the part and thus increase the fiber volume fraction prior to curing it.
However, these vacuum bag methods are obviously not satisfactory for making a composite part with a lower volume fraction of reinforcement fiber. Moreover, adding a large excess of resin to a wet resin or prepreg lay-up to produce a lower fiber volume fraction generally results in a poor quality part that is hard to reproduce and that typically has a high void content. Although some control over the fiber volume fraction can be obtained by using a fabric with a different weave pattern, for example, a 0-90-stitched fabric as opposed to a plain weave fabric, this technique is also generally inadequate to produce composites having low fiber volume fraction.
Vacuum assisted resin transfer molding (VARTM) or variations on this process, result in a fiber volume fraction determined by the applied pressure and the fabric type. VARTM can be broken down into five basic steps. In the first step, a dry preform is manufactured which conforms to the shape of the tool or rigid plate on which the part is produced. The second step is to place the preform on the rigid side of the tool. In the third step, the preform is sealed against the tool with a vacuum bag. Thus, the difference between traditional Resin Transfer Molding (RAM and VARTM is two-sided versus one-sided tooling, respectively. The vacuum bag is a membrane that is used to allow a vacuum to be drawn, meaning that the preform will be evacuated. Next, the thermosetting polymer resin is infused into the preform. The process uses the vacuum on the inside and atmospheric pressure outside to provide the pressure gradient required for flow. The vacuum also provides for the consolidation pressure. This is usually performed at room temperature, although, provided there is a big enough oven, elevated temperature can be used. Next, the part is cured; this also can be done at either room or elevated temperatures depending on the requirements of the resin. As can be expected, the resin system selected plays a role in the processing conditions, and this does limit the resins available for use in VARTM. Finally, the cured near net shape part is removed from the tool and any post processing can be done. In addition, some finishing operations may be required, in that there is only one good surface finish supplied by the mold and the other surface is a vacuum bag surface. The most popular type of VARTM currently in use is Seeman""s Composite Resin Infuision Molding Process (xe2x80x9cSCRIMPxe2x80x9d(copyright)), which is described in U.S. Pat. Nos. 4,902,215; 5,052,906; 5,316,462; 5,439,635; 5,601,852; and 5,702,663; hereby expressly incorporated by reference.
The degree of control over the fiber volume and part thickness obtained by methods that do not use a closed mold is minimal, however. Resin transfer molding is often an alternative to vacuum bag methods. RTM can be broken down into the same basic five basic steps, as in VARTM, except that a two-part rigid mold or tool is used. The injection is performed under heat and pressure, typically the resin may be injected anywhere from 15 psi to 250 psi. During the cure cycle, the resin will react and polymerize into a solid structure. This reaction may need increased heat and pressure to react in an optimum fashion. Finally, the cured near net shape part is removed from the tool and any required post processing can be done. The finishing operations are minimal due to the near net shape of the component and the good surface finish supplied by the mold. The volume fraction of fiber in a composite part made by the RTM method can be controlled by the amount of compression the mold provides to the dry fabric. The mold also determines the part thickness. However, this process requires a relatively expensive closed mold, which could be an unacceptable expense if only a few parts are needed.
The present invention provides a reproducible method of fabricating composite parts having a low volume fraction of reinforcement fiber and carefully controlled thicknesses, and does so using inexpensive vacuum bag methods as opposed to closed molds.
It is an object of the present invention to provide a repeatable and inexpensive method of fabricating polymer resin matrix composite parts having a controlled, low volume percentage of fiber reinforcement.
It is another object of the present invention to provide an inexpensive method of fabricating polymer matrix composite parts having carefully controlled thicknesses.
It is still another object of the present invention to provide an inexpensive method of producing polymer resin matrix composite parts having reinforcement fiber volume fractions less than 0.50 and without objectionable void content.
It is yet another object of the present invention to provide a vacuum bag method of fabricating composite parts which prevents the common xe2x80x9cracetrackingxe2x80x9d phenomena of resin flow, i.e., the flow of resin around the periphery of the fabric stock rather than through it.
Accordingly, this and other objects of the present invention are achieved by using standard vacuum assisted resin transfer molding (VARTM) techniques with simple and inexpensive improvements. In particular, rigid supports or spacers, typically metal bars or rods, are placed on two opposite sides of the fabric stack. The fill line and vacuum line are then placed on the opposite unsupported sides of the assembly. Finally, a rigid cover plate, also typically metal, is placed over the assembly so that it rests on the supports. Adding these elements to a typical vacuum bag molding system enables fabrication of parts having a low volume percentage of reinforcement fiber and controlled thickness. In addition, xe2x80x9cracetrackingxe2x80x9d can be prevented by cutting the fabrics to a particular pattern and supporting the cover plate in a slightly different manner as will be disclosed herein.
Further objects, features and advantages of the present invention will become apparent from the following detailed description taken in conjunction with the accompanying drawings.